ALBERT

Notes: We have measured and assigned more than 800 new far-infrared absorption lines and 12 new microwave absorption lines of the ammonia dimer. Our data are analyzed in combination with all previously measured far-infrared and microwave spectra for this cluster. The vibration–rotation–tunneling (VRT) states of the ammonia dimer connected by electric-dipole-allowed transitions are separated into three groups that correspond to different combinations of monomer rotational states: A+A states (states formed from the combination of two ammonia monomers in A states), A+E states, and E+E states. We present complete experimentally determined energy-level diagrams for the Ka=0 and Ka=1 levels of each group in the ground vibrational state of this complex. From these, we deduce that the appropriate molecular symmetry group for the ammonia dimer is G144. This, in turn, implies that three kinds of tunneling motions are feasible for the ammonia dimer: interchange of the "donor'' and "acceptor'' roles of the monomers, internal rotation of the monomers about their C3 symmetry axes, and quite unexpectedly, "umbrella'' inversion tunneling.In the Ka=0 A+E and E+E states, the measured umbrella inversion tunneling splittings range from 1.1 to 3.3 GHz. In Ka=1, these inversion splittings between two sets of E+E states are 48 and 9 MHz, while all others are completely quenched. Another surprise, in light of previous analyses of tunneling in the ammonia dimer, is our discovery that the interchange tunneling splittings are large. In the A+A and E+E states, they are 16.1 and 19.3 cm−1, respectively. In the A+E states, the measured 20.5 cm−1 splitting can result from a difference in "donor'' and "acceptor'' internal rotation frequencies that is increased by interchange tunneling. We rule out the possibility that the upper state of the observed far-infrared subbands is the very-low-frequency out-of-plane intermolecular vibration predicted in several theoretical studies [C. E. Dykstra and L. Andrews, J. Chem. Phys. 92, 6043 (1990); M. J. Frisch, J. E. Del Bene, J. S. Binkley, and H. F. Schaefer III, ibid. 84, 2279 (1986)]. In their structure determination, Nelson et al. assumed that monomer umbrella inversion tunneling was completely quenched and that "donor–acceptor'' interchange tunneling was nearly quenched in the ammonia dimer [D. D. Nelson, G. T. Fraser, and W. Klemperer, J. Chem. Phys. 83, 6201 (1985); D. D. Nelson, W. Klemperer, G. T. Fraser, F. J. Lovas, and R. D. Suenram, ibid. 87, 6364 (1987)]. Our experimental results, considered together with the results of six-dimensional calculations of the VRT dynamics presented by van Bladel et al. in the accompanying paper [J. Chem. Phys. 97, 4750 (1992)], make it unlikely that the structure proposed by Nelson et al. for the ammonia dimer is the equilibrium structure.

Notes: The first far infrared intermolecular vibration–rotation spectrum of the ternary van der Waals cluster has been measured near 39.5 cm−1 and assigned to an a-type ∑ bending vibration of Ar2HCl. Spectra of both chlorine isotopes were observed and nuclear quadrupole hyperfine structure was resolved. Values of the fitted constants (rotational constants, hyperfine projections) evidence large amplitude out-of-plane motion, and demonstrate the sensitivity of spectroscopic observables to the three body forces operative in the Ar2HCl system. Spectroscopic predictions calculated by Hutson et al. from pairwise-additive and "three-body'' corrected potential energy surfaces [J. Chem. Phys. 90, 1337 (1989)] are compared to experimental results.

Notes: A second Ar2HCl intermolecular vibration–rotation band centered at 37.2 cm−1 has been measured and assigned as a b-type transition originating from the ground state. Nuclear hyperfine splittings were resolved for both chlorine isotopes. The rotational constants determined from the data indicate coupling between an Ar–Ar stretching or bending coordinate and the Ar2 –HCl vibrational coordinates. As a result of this particular vibrational motion, Ar2H 35Cl undergoes an axis-switching transition while the Ar2H 37Cl isotope does not. In addition, the measured hyperfine projections indicate the possibility of coupling between the Ar2 –HCl stretching and bending modes, preventing an absolute vibrational assignment. These results indicate that the "reversed adiabatic'' approximation employed by Hutson, Beswick, and Halberstadt in their theoretical study of Ar2HCl [J. Chem. Phys. 90, 1337 (1989)] is not appropriate for the complicated intramolecular dynamics presently observed in this system.

Notes: The Fourier transform microwave spectrum of the propane–water complex (C3H8–H2O) has been observed and analyzed. This spectrum includes transitions assigned to propane complexed with both the ortho and para nuclear spin confirmations of water. The rotational constants indicate that the vibrationally averaged structure has all four heavy atoms coplanar, with the water center of mass lying on or near the C2 axis of propane, inside the CCC angle, 3.76(±0.02) A(ring) from the propane center-of-mass, and 4.35(±0.02) A(ring) from the methylene carbon. The projection of the electric dipole onto the a inertial axis of the complex (0.732 D for the ortho state and 0.819 D for the para state) indicates that one of the protons of the water subunit lies on the C2 axis of the propane monomer, which is also the axis connecting the subunit centers of mass. The small projection of the dipole along the b axis (0.14 D for the ortho state and 0.38 D for the para state) is most consistent with an equilibrium structure in which all three atoms of the water lie in the CCC plane of propane, with torsional tunneling about the hydrogen bond occurring on the same time scale as the overall rotation. The small internal rotation tunneling splittings that occur in the rotational spectrum of the propane monomer are not observed in the spectrum of the complex.

Notes: The c-type intermolecular out-of-plane bend of Ar2HCl has been observed at 45.2 cm−1, completing the high resolution far infrared measurements of the three lowest-lying Ar2HCl bending states which correlate to the j=1 internal rotational state of the HCl monomer. The rotational and nuclear quadrupole hyperfine structures indicate the existence of a Coriolis perturbation. The perturbing state is postulated to be a heavy-atom stretching overtone that is very nearly degenerate with the out-of-plane bend. A partial reassignment of the previously reported [J. Chem. Phys. 95, 3182 (1991)] Ar2HCl in-plane bend is presented and a treatment of Coriolis effects between the in-plane and Σ bends is discussed. Comparison with dynamically rigorous calculations presented in the accompanying paper [J. Chem. Phys. 98, 5337 (1993)] indicate substantial three-body contributions to the intermolecular potential, which should be determinable from the data presented in this paper.

Notes: The far-infrared laser electric resonance spectra of the prototypical atom–diatom complex ArH35Cl are analyzed using improved zero-field molecular constants, yielding accurate permanent and transition dipole moments for the three lowest excited van der Waals vibrational states. The constants are obtained from a multistate fit to previous microwave, far-infrared laser electric resonance, and far-infrared tunable laser spectra, as well as new far-infrared measurements of the Σ-stretch state, which are reported here. The signs of the dipole moments and Coriolis coefficients establish the relative orientations of the HCl subunit in these states. The fit is found to converge only if these signs correspond to the HCl pointing in opposite directions along the a inertial axis in the Σ-stretch and Σ-bend states. A weak preference, near the experimental error limit, is found for the Ar–Cl–H average angle in the Π-bend state to be greater than 90°, contrary to expectation. For the best fit, we obtain the a-axis dipole moment components −0.5413(11) D (Σ bend), −0.263 45(29) D (Π bend), and 0.6754(36) D (Σ-stretch); and the b-axis components 0.365(12) D (Π and Σ-bend) and −0.0465(43) D (Π and Σ stretches), where the signs of the Coriolis coefficients and μa for the Σ stretch have been arbitrarily fixed positive. For the expected Π-bend configuration, with the Ar–Cl–H angle less than 90°, the magnitudes along the a axis change only slightly, but the b-axis components become 0.149(12) and −0.1403(64) D for the Π–Σ-bend and Π–Σ-stretch interactions, respectively.

Notes: Several commonly used approximate methods for the calculation of vibration–rotation–tunneling spectra for (HCl)2 are described. These range from one-dimensional models to an exact coupled four-dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed—an ab initio and our ES1 experimental surface (determined by imbedding the four-dimensional calculation outlined here in a least-squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface. © 1995 American Institute of Physics.

Notes: An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least-squares fit to 33 microwave, far-infrared and near-infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four-dimensional dynamical method (described in the accompanying paper). The global minimum (De=−692 cm−1) is located near the hydrogen-bonded L-shaped geometry (R=3.746 A(ring), θ1=9°, θ2=89.8°, and φ=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm−1) process involves a contraction of 0.1 A(ring) in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)]. © 1995 American Institute of Physics.

Notes: Thirteen vibration-rotation-tunneling (VRT) bands of the CH4–H2O complex have been measured in the range from 18 to 35.5 cm−1 using tunable far infrared laser spectroscopy. The ground state has an average center of mass separation of 3.70 A(ring) and a stretching force constant of 1.52 N/m, indicating that this complex is more strongly bound than Ar–H2O. The eigenvalue spectrum has been calculated with a variational procedure using a spherical expansion of a site–site ab initio intermolecular potential energy surface [J. Chem. Phys. 93, 7808 (1991)]. The computed eigenvalues exhibit a similar pattern to the observed spectra but are not in quantitative agreement. These observations suggest that both monomers undergo nearly free internal rotation within the complex.